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## Chemistry library

### Course: Chemistry library>Unit 15

Lesson 2: Enthalpy

# Enthalpy

Understanding why enthalpy can be viewed as "heat content" in a constant pressure system. Created by Sal Khan.

## Want to join the conversation?

• I am still having a problem with enthalpy v.s. internal energy. To better understand the relationship between the two, is it correct in thinking that the two are opposites? Meaning: if delta U = +10J then delta H = -10J •   internal energy change is the heat absorbed or evolved at constant VOLUME
q = deltaU +P deltaV
since delta V =0
q=deltaU
......................................................................
enthalpy change equal to the haet absorbed or evolved by the systemat constant PRESSURE
Q=DELTA U-W
Q= delta U + P deltaV
Q= (U2-U1) +p(V2-V1)
or, Q= (U2 +PV2)- ((U1+PV1)
Q = H2-H1=DELTA H
.....................................................
HOPE THE ANSWER IS CLEAR NOW!!!..:)
• I don't understand the claims at :( . There is an equation written :
delta P * V = P * delta V
and Sal says if the pressure is constant, these will cancel out.
Pressure being constant means: delta P = 0. But P still has a value.
So filling it in gives:
0 * V = P * delta V
0 = P * delta V (because P does have a value, even though it's constant)
But P has a value, so the right term IS NOT 0 (unless delta V is 0, or P = 0).
So i dont know how both terms cancel out if only the pressure is constant T_T, help? • Just to clarify further:
delta(P.V)=delta(P).V + P.delta(V)

Now, when Sal says that pressure is not changing, is constant, you can see that delta(P)=0 so that the first term is dropped, leaving...
delta(P.V)=P.delta(V)
• What exactly is the U standing for? • How does enthalpy relate to calorimetry? • Calorimetry is the science of measuring heat and Enthalpy is the internal energy in the system added to the product of the pressure and volume. At a constant pressure the heat is the same as the enthalpy. Enthalpy is very closely related to heat that is why they relate to each other.
• What is a state variable? •  In thermodynamics, a state variable is a property of a system that depends only on the current equilibrium state of the system
Examples of state variables are temperature, pressure, volume, internal energy, enthalpy, and entropy.
A state variable does not depend on the path by which the system arrived at its present state.
For example, if I have 1 L of a gas, I can heat it, cool it, compress it, expand it, etc. But if I end up with a volume of 1 L, the volume is the same as it was at the beginning.
So, volume is a state variable.
• Can absolute enthalpy and absolute internal energy be determined ? • In theory - but it would take more than a lifetime. For instance, internal energy is the kinetic energy of all the particles (and all the other energy in a system, but lets assume this is an ideal gas). It would take forever to calculate the kinetic energy of every particle because they are probably all different, and there are millions of them. You would need a super computer and that would require a lot of energy, because the particles kinetic energy will change when they collide with each other so the computer would have to calculate it instantly.

So in theory? Yes. But completely impossible in practice.
• I don't get why ΔPV = PΔV at constant pressure. If pressure is constant, then wouldn't ΔP be 0 so ΔPV would be 0? • So is Work = Pressure * Volume, Change in Pressure * Volume, Pressure * Change in Volume or Change in Pressure * Change in Volume? • Work = Pressure * change in Volume, because by defnition Work = Force*distance. Well we know Pressure = Force/Area. Solving for Force, force= area*pressure. We also know that distance, lets say x is how far an area traveled. If we set it up in 3D like a cylinder, we see that the change in distance is the height of cylinder. Volume = Area*height, but since height is changing, so is volume.  